Battery systems are playing an increasing role as energy sources in power grids. The energy available from a battery system is typically understood through a round-trip efficiency (RTE) metric, which is a product of one-way charge and discharge efficiencies. Conventional approaches of determining these metrics, however, typically rely on information about a battery system that is not always available. For example, to determine one-way efficiency for charge or discharge within a state-of-charge range at a particular temperature, DC efficiency for the battery and inverter efficiency typically must be known. Even when such information is available, conventional approaches of determining these metrics result in overly simplistic approximations that can be inaccurate, thus limiting the potential of widespread deployment of battery systems in power grids.